1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory, and more particularly, to a memory of a metal ferroelectric silicon (MFS) field effect transistor type.
2. Description of the Prior Art
A conventional MFS field effect transistor (FET) of this type is of a structure as shown in FIG. 1 where a ferroelectric thin film 103 is directly formed on a silicon substrate 100 between a source region 101 and a drain region 102 formed in the substrate 100, and a metallic thin film 104 is provided on the ferroelectric thin film 103.
However, this structure creates the following problems: First, since there are no ferroelectric substances having a lattice constant which is matched with that of silicon, the mismatch between the lattice constant of the silicone substrate 100 and that of the ferroelectric film 103 is large, so that a ferroelectric thin film having an excellent crystallizing capability cannot be formed. In addition, since mainly used ferroelectric substance is an oxide, an unnecessary film such as an SiO.sub.2 film having a low dielectric constant is formed between the substrate 100 and the ferroelectric thin film 101 when the film 101 is formed (600.degree. C.). This unnecessary film deteriorates the film quality, increases the operating voltage and increases the trap level. The increase in operating voltage is caused because it is necessary for the operating voltage to be increased since the electrical field is applied mostly to the low-dielectric-constant SiO.sub.2 film (the unnecessary film) and barely to the ferroelectric thin film 103 when a voltage is applied. The increase in trap level is caused because a charge is applied to the unnecessary film to countervail the polarization of the ferroelectric substance.
Moreover, in the above-described conventional semiconductor memory, the elements composing the ferroelectric thin film 103 are dispersed in the substrate 100 to break the balance of the charge in the channel portion, so that a desired FET characteristic cannot be obtained. For these reasons, an MFS FET device using the conventional memory has not been put into practical use. Further, in the conventional semiconductor memory, as shown in FIG. 2, two selector transistors 106 and 107 are necessary for reading and writing for every MFS FET 105, so that the area of a memory cell 108 increases.